1. Field of the Invention
The present invention relates to a communication system, and more particularly a coupler for preventing divergence of a phase detection voltage normalized for diversity operation.
2. Description of the Related Art
Generally in shortwave radio communications, the electric field strength of a receiving point is irregular with time due to the fading effect which results in unstable communication. In order to prevent such phenomenon, a diversity system is employed, in which the outputs of two or more antennas in different locations are combined into a single received signal for obtaining a stable output. Accordingly, a signal coupler is needed to combine signals received by two or more antennas in the diversity operation mode.
In an earlier coupler, first and second signals from first and second antennas are input to the coupler.
A phase shifter splits the first input signal into signals having a phase of 0.degree. and phase of 90.degree.. A first multiplier multiplies the 0.degree. phase signal by the second input signal while a second multiplier multiplies the 90.degree. phase signal by the second input signal. The outputs of the first and second multipliers are respectively fed to first and second low pass filters whose output is fed to a normalization circuit.
One output of the normalization circuit is multiplied by the 0.degree. phase signal in another multiplier while a second output of the normalization circuit is multiplied by the 90.degree. phase signal in still another multiplier. The outputs of these two multipliers are added in an adder and the output of the adder is added to the second input signal in another adder whose output forms the combined value of two signals of the same intensity after adjusting the two signals so as to be equiphase.
The normalization circuit includes two squaring circuits whose outputs are summed together in an adder. The output of the adder is fed to a square root circuit whose output is fed to a pair of dividers whose other inputs are respectively fed to the two inputs to the normalization circuit.
Normally, the output of the square root circuit is a constant value. However, if the two inputs fed to the normalization circuit are very low, the normalized signal output from the square root circuit becomes very low, thereby resulting in a divergences of the phase detection signals having a constant vector sum and output from the normalization circuit. Consequently, the phase shifing performance is effected by the divergence of the phase detection signals, thereby causing a voltage fluctuation phenomenon.
The following patents are but a sample of diversity systems having features in common with the present invention but nevertheless failing to teach or suggest a coupler including elements for preventing divergence of the phase detection voltage normalized for diversity operation.
U.S. Pat. No. 5,539,781 to Matsuura, entitled Combining Diversity Apparatus With Squelch Function, U.S. Pat. No. 5,465,271 to Hladik et al., entitled Post Detection Weighted Vector Combining Diversity Receivers Using Phase Metrics For Mobile And Indoor Radio Channels, U.S. Pat. No. 5,426,668 to Lechleider, entitled Tetherless Access To Communication Networks, U.S. Pat. No. 5,297,168 to Sant'Agostino et al., entitled Space-Diversity Digital Mobile Receiver And Relevant Process, U.S. Pat. No. 5,321,850 to Backstrom, et al., entitled Diversity Radio Receiver Automatic Frequency Control, U.S. Pat. No. 4,731,801 to Henriksson, entitled Method For The Reception And Detection Of Digital Signals, and U.S. Pat. No. 4,270,207 to Gutleber, entitled Combined ECCM/Diversity Topospheric Transmission System.